This invention relates to the preparation of polycarbonates, and more particularly to a method for their preparation by solid state polymerization.
Solid state polymerization is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073, the disclosures of which are incorporated herein. It involves three steps: a first step of forming a prepolymer, typically by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate; a second step of crystallizing the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. Use of this method is of increasing interest by reason of its effectiveness and environmental benefits.
The aforementioned U.S. Pat. No. 5,204,377 describes a solid state polymerization method which requires the use of a crystallized prepolymer having a specific surface area of at least 0.2 m.sup.2 /g. This is a rather high surface area and is difficult to achieve, requiring, for example, high shear conditions. It is of interest, therefore, to develop a solid state polymerization method which does not require crystallized prepolymers having such a high surface area.